There is a need for introducing a foreign protein in an aquatic animal. Such a foreign protein may lead to an efficacious immunization of aquatic animals. For example, infectious diseases are common on fish farms, due to intensive fish farming that facilitates the transmission of pathogens in an aqueous environment. Dunn et al. (1990) Aquaculture Engineering 9: 23. Preventing aquatic animal diseases by oral vaccination has several advantages over other methods: It is non-stressful, requires little labor, and can be applied at a large scale. However, many oral vaccines have been found ineffective as a result of failure to uptake sufficient dosage of antigen, poor antigen delivery and antigen degradation in the digestive tract.